Obesity is associated with dysfunction of adipose tissue due to oxidative stress and
inflammation, leading to insulin resistance. Thiosulfate sulfurtransferase (Tst) was
previously identified as an adipose-expressed anti-diabetic gene that protects against
diet-induced metabolic impairment when upregulated in adipose tissue of mice. TST
is a mitochondrial enzyme involved in the metabolism of cyanide, reactive oxygen
species (ROS) and endogenous hydrogen sulfide (H2S). This thesis tested the
hypothesis that TST maintains metabolic health in the face of dietary obesity. To do
this, I investigated the adipose-tissue phenotypes and metabolic consequences of Tst
gene deletion (Tst–/– mice) and of adipose tissue-specific overexpression of human
TST (Ad-hTST mice) after exposure to high fat diet (HFD).
After 20 weeks of HFD, Tst–/– mice exhibited impaired glucose tolerance
despite unchanged adipose tissue inflammatory cell infiltration, protein carbonylation
and unfolded protein response activation. However, levels of mRNA encoding
mitochondrial antioxidant enzymes including superoxide dismutase 2 and
peroxiredoxin 3 were lower in Tst–/– mice on HFD. Unexpectedly, chow-fed Tst-/- mice
had lower body weight and fat mass than wild-type controls highlighting a potential
effect of Tst on fat accumulation with age.
A new mouse model with high expression of human TST genetically targeted
to adipose tissue (Ad-hTST) was developed using the LoxP / Cre recombinase
expression system, with a parent line expressing Cre under the control of the
adiponectin promoter to confer adipose specificity. The Ad-hTST mice were found to
gain a similar amount of weight and fat mass to control mice when exposed to 6 weeks
of HFD. However, Ad-hTST mice had impaired glucose tolerance with no change in
inflammatory cell infiltration, mRNA levels of antioxidant enzymes or unfolded
protein response genes. Thus, unexpectedly, overexpression of human TST in adipose
tissue of mice results in a detrimental metabolic phenotype.
In vivo and in vitro experiments were conducted to test the hypothesis that TST
protects against ROS accumulation. Paraquat was tested as an inducer of oxidative
stress in vivo in wild-type, Tst-/- and Tst+/- mice. At the doses used (25mg/kg and
under), mice became unwell and lost weight, with no increase in markers of oxidative
stress in adipose or lung. The production of mitochondrial ROS in response to
exogenous hydrogen peroxide (H2O2) exposure was increased in primary adipocytes
from Tst-/- mice in vitro. However, primary hepatocytes showed reduced mitochondrial
ROS production in response to H2O2 exposure. ROS production in hepatocytes was
unaffected by pre-incubation with a H2S donor, an inhibitor of H2S-producing enzyme
CSE or N-acetyl-cysteine, an antioxidant. TST may therefore influence mitochondrial
ROS production differently in cell types such as adipocytes and hepatocytes. Disposal
of exogenous H2O2 was unchanged in primary adipocytes from Tst-/- and Ad-hTST
mice, and this was not affected by pre-incubation with sodium thiosulfate, a TST
substrate.
Metabolic changes in response to HFD may be influenced by alteration in TST
expression, however the current data suggest it is unlikely to occur through the
prevention of excessive local ROS accumulation in adipose tissue. Mice lacking the
Tst gene globally and mice with adipose-specific overexpression of the human TST
gene have a similarly impaired metabolic response to HFD. The phenotype of adipose-specific
human TST-overexpressing mice does not recapitulate the protective
metabolic phenotype produced by overexpression of the endogenous mouse Tst gene.
In conclusion, TST may influence adipose tissue due to its role in the oxidation of H2S,
however, by the current means, it does not appear to substantially impact the response
of this tissue to oxidative stress